containers: an expired token could never be replaced

Credentials were seeded only when the container's copy was MISSING. So when a
session expired, the file still existed, the copy was skipped, and restarting
changed nothing -- the one recovery path a human has, re-logging in on the host,
could not reach the containers at all.

Now re-seeds whenever the host's copy is newer. Newer-wins rather than
always-copy, because a container refreshes its own token mid-run and that copy
may legitimately be the fresher of the two.

Found when both sessions expired: host credentials at 16:30, containers holding
14:20 and 14:24.

Cargo.lock

@@ -4618,6 +4618,18 @@ dependencies = [
  "tracing-subscriber",
 ]
 
+[[package]]
+name = "sylpheed-export"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "clap",
+ "image",
+ "serde",
+ "serde_json",
+ "sylpheed-formats",
+]
+
 [[package]]
 name = "sylpheed-formats"
 version = "0.1.0"